1. Field of the Invention
The present invention relates generally to the operation of batteries and, more specifically, to the manner of recharging batteries such as those used in spacecraft applications. The invention serves to extend the service life of batteries, especially those employed for powering long life geosynchronous spacecraft or in other instances in which there is a high degree of predictability about the operation of a battery.
2. Description of the Prior Art
Lithium ion batteries which are desirable for use in powering many spacecraft systems typically recharge with voltages between 3.0 and 4.5 volts. Battery voltage increases as the state of charge of the battery is increased. Thus the more charge stored in the battery, the higher its recharge voltage. High voltages can lead to parasitic reactions at both the positive and negative electrodes. Typical positive electrodes for lithium ion batteries are metal oxides such as CoO.sub.2, NiO.sub.2, and MnO.sub.2, As the voltage of these metal oxides is increased their thermodynamic stability versus oxygen decomposition, e.g., EQU M.sub.x O.sub.y .fwdarw.M.sub.x.O.sub.y-2 +O.sub.2 ( 1)
decreases. In addition recharge of the positive electrode to higher voltages increases its power as an oxidizing agent. Electrolyte in contact with the positive electrode is thus more likely to be destructively oxidized by the positive electrode. An example of such a reaction in aqueous batteries is the electrolysis of water at a nickel electrode during recharge. In the aqueous example the reaction is reversible and is not believed to strongly effect battery life. In a non-aqueous battery employing organic electrolytes a similar oxidation reaction is irreversible and will shorten the battery life.
Similarly, during recharge of the negative electrode, it becomes a stronger reducing agent as its state of charge is increased. As it is also in contact with the non-aqueous electrolyte, the tendency of the electrolyte in contact with the negative electrode to be irreversibly reduced increases. This problem is further exacerbated in lithium ion battery where the carbon or graphite lithium negative electrode may as the state of charge increases approach the voltage of lithium metal. In this case during recharge lithium metal may be deposited on the surface of the carbon based electrode which in turn will lead to a direct and irreversible reaction between lithium metal and the non-aqueous electrolyte.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.